Sterilisation indicator

ABSTRACT

The invention provides a sterilization indicator ( 17 ), characterized in that it comprises:
         a compound ( 21 ) that is of the heating type when put into contact with atoms of oxygen O and/or nitrogen N; and   a thermochromic dye ( 19 ), in thermal contact with the compound ( 21 ).

The present invention relates to a sterilization indicator, usable inparticular for sterilizing medical or surgical instruments, inparticular for dental use, by means of a post-discharge plasma obtainedfrom a gas based on nitrogen and/or oxygen.

Sterilization consists in destroying a significant number, in a givenproportion, of microorganisms, viruses, or pathogenic proteins presenton the inside or outside surfaces of articles to be sterilized.

It should be observed that the adjective “sterile” is an absolute term,whereas ensuring that an article is sterile, i.e. free frommicroorganisms, is a probability function. The sterility assurance level(SAL) of an article is defined as the probability that any given unit isnot sterile after being exposed to a validated sterilization process.Thus, for an article to be considered as being sterile with respect toEuropean standard EN556, it must possess an SAL of 10⁻⁶, i.e. thetheoretical probability of isolating a microorganism must be less than 1in 10⁶.

Various sterilization processes are known, including chemical andphysical processes.

Amongst chemical sterilization processes, mention can be made ofprocesses making use of gases such as ethylene oxide, formaldehyde, orhydrogen peroxide. Those processes nevertheless present the drawback ofrequiring a long period of time for desorption, and that is incompatiblewith the instruments for sterilizing being made rapidly available. Inaddition, those gases are toxic and irritate the skin and the mucousmembranes.

Physical processes include in particular steam sterilization in anautoclave under the action of temperature and steam; dry hotsterilization; radiation using ion beams or gamma rays, used on articlesthat cannot be sterilized by means of heat or chemically; or indeedfiltering using filters that are suitable for separating outmicroorganisms. Those processes generally involve high temperatures,often higher than 100° C.

Because of the increasing use in medical articles of materials that aretemperature-sensitive, such as polymer-based materials, it is desirableto develop sterilization processes usable at low temperature, and inparticular at temperatures lower than 70° C.

Sterilization processes have thus been developed that makes use of aplasma, operating at temperatures that makes it possible to avoiddamaging temperature-sensitive materials.

For example, application WO 00/72889 discloses a sterilization processthat uses a plasma based on oxygen and nitrogen. Application FR 2 856600 describes a sterilization process making use of a post-dischargeplasma coming from a plasma made exclusively of nitrogen, whileapplication FR 2 879 933 uses a post-discharge plasma coming from aplasma made up of hydrogen and of nitrogen. Numerous sterilizationprocesses thus make use of a gas based on nitrogen or oxygen forpreparing the plasma.

In order to validate sterilizers, use is made of sterilizationindicators that make it possible to monitor one or more essentialparameters of the sterilization process. For this purpose, three typesof sterilization indicator have been developed: physical, chemical, andbiological indicators.

Sterilization indicators for processes making use of a plasma are stilllittle developed. Documents U.S. Pat. No. 6,659,036, WO 98/46279, JP2005111154, and JP 2004298479 disclose sterilization indicators relatingto processes that make use of plasma. Nevertheless, those documents donot relate to plasmas based on nitrogen and/or oxygen.

It thus appears necessary to have sterilization indicators availablethat enable at least one parameter of the sterilization process to beevaluated for sterilization processes making use of a post-dischargeplasma obtained from a gas comprising nitrogen N₂ and/or oxygen O₂.

The Applicant has developed a sterilization indicator enabling thisobjective to be achieved. In particular, the indicator of the inventionenables the presence of atoms of oxygen O and/or nitrogen N to beindicated in a post-discharge plasma.

The sterilization indicator of the invention, usable in a sterilizationdevice, comprises:

-   -   a compound of the type that heats on being put into contact with        atoms of oxygen O and/or nitrogen N, in particular those present        in a post-discharge plasma obtained from a gas comprising oxygen        O₂ and/or nitrogen N₂; and    -   a thermochromic dye, in thermal contact with the compound.

The term “in thermal contact with the compound” in the meaning of theinvention designates a dye that is placed relative to the compound insuch a manner that heating of the compound causes the temperature of thedye to rise.

The compound is preferably in the form of a powder, particles,filaments, or fibers, in order to ensure a large contact area betweenthe compound and the dye, thereby enhancing heat exchange.

The compound preferably comprises at least one metal. Surfacerecombination of atoms of nitrogen N or hydrogen H, which is exothermal,is enhanced by the presence of metal surfaces. The probability of atomicrecombination of atoms of nitrogen N or oxygen O is large on metalsurfaces. Since this reaction is exothermal, the reaction energy of therecombination is transferred in part to the metal.

The metal of the compound may be selected from copper, titanium, steels,aluminum, and alloys thereof.

It is preferable to use copper and its alloys.

The thermochromic dye may be reversible or irreversible. A thermochromicdye is a dye that changes color under the action of temperature. It isirreversible when the color obtained under the action of temperature ispermanent over time and does not return to its original color.

The reversible or irreversible nature of the thermochromic dye isselected as a function of the use intended for the indicator. If it isdesired to track changes in the color of the dye in real time and tomake use of it again quickly after sterilization, then a reversiblethermochromic dye can be used. In contrast, if the indicator is to bestored with the articles for sterilization for the purpose of subsequentverification of sterilization parameters, then irreversiblethermochromic dye should be used.

The dye should be selected as a function of the nature of the compoundand of the parameters of the sterilization process. The temperature riseof the dye is a function of the capacity of the compound to heat up incontact with atoms of nitrogen N and/or oxygen O, and also of thequantity of atoms of nitrogen N and/or oxygen O with which the compoundis put into contact.

The compound and the dye are preferably in contact with a medium, i.e.disposed on and/or in the medium. The compound may thus be disposed ordispersed on a medium on which the dye is disposed. The compound mayalso be disposed inside the medium, e.g. by spraying or by interlacingin the form of fibers in the medium.

Any type of medium may be used, e.g. a medium that is porous, in orderto facilitate diffusion of the dye in the medium.

The medium may be in the form of a plate, e.g. made of polymer or ofmetal, in the form of fibers, e.g. metal, natural, or chemical fibers,or indeed in the form of woven or non-woven fabrics.

In particular, the medium may be selected from cellulose, fabric,fibers, cotton, paper, and blotting paper.

The invention also provides a method of indicating the presence of atomsof oxygen O and/or nitrogen N in a plasma present in a post-dischargechamber (post-discharge plasma). The method is advantageouslyimplemented in a post-discharge chamber of a sterilization device usinga plasma obtained from a gas comprising oxygen O₂ and/or nitrogen N₂.The method comprises in succession:

-   -   putting a sterilization indicator of the invention into contact        with the plasma; and    -   comparing the color of the dye of the indicator with a reference        color.

The reference color is set as a function of parameters used during thesterilization process.

Thus, by comparing the color of the dye after the process with thereference color, it is possible to observe whether the indicator of theinvention has been subjected to the desired quantity of atoms ofnitrogen N and/or oxygen O. The heating of the compound, and thus itstemperature, are proportional to the quantity of atoms of nitrogen Nand/or oxygen O with which the compound is in contact. Since thetemperature reached by the compound determines the color of the dye, thecolor observed is indeed associated with the quantity of atoms ofnitrogen N and/or oxygen O with which the compound has been in contact.

The color of the indicator of the invention can be compared with thereference indicator visually or by using automatic comparator means,such as a spectrophotometer, for example.

In order to define whether two colors are considered to be identical, itis possible to use the Kubelka-Munk equation and color variation inaccordance with the CIE Lab model a model for representing colors asdeveloped by the International Commission on Illumination (CIE).

From the Kubelka-Munk equation, the ratio between the absorptioncoefficient K of the dye and the diffusion coefficient S of the dye isassociated with the reflection coefficient β as follows:

K/S=(1−β)²/(2β)

According to the CIE Lab model, the color difference between twocompounds 1 and 2 is as follows:

$\begin{matrix}{{\Delta \; E_{ab}^{*}} = \left( {\left( {L_{2}^{*} - L_{1}^{*}} \right)^{2} + \left( {a_{2}^{*} - a_{1}^{*}} \right)^{2} + \left( {b_{2}^{*} - b_{1}^{*}} \right)^{2}} \right)^{1/2}} \\{= \left( {{\Delta \; L^{*_{2}}} + {Da}^{*2} + {Db}^{*_{2}}} \right)^{1/2}}\end{matrix}$

in which equation, L* is luminance, a* represents position on themagenta->green axis, and b* represents position on the yellow->blueaxis.

For the colors of two dyes to be considered as being identical, it isthus possible to require ΔE*_(ab)≦2.

In one implementation, the indicator can also be used to indicatetemperature in the post-discharge chamber. Under such circumstances, itis appropriate to observe the color of the dye when the indicator isplaced in the post-discharge chamber, at the beginning of thesterilization process. The heating of the compound under the effect ofcontact with atoms of nitrogen and/or oxygen is slower than the changein the color of the dye associated with the temperature that exists inthe post-discharge chamber.

In this implementation, it is preferable to select a thermochromic dyein which color change is progressive as temperature goes from ambienttemperature (20° C.) to the temperature to which the metal is heated, soas to present at least two distinct colors.

Such a dye may in particular be a dye from Chemsong Inc.

The invention thus provides a method of indicating a) the presence ofatoms of oxygen O and/or nitrogen N in a plasma present in apost-discharge chamber, possibly being obtained from a gas comprisingoxygen O₂ and/or nitrogen N₂; and b) the temperature in thepost-discharge chamber.

The method comprises in succession:

-   -   putting a sterilization indicator of the invention into contact        with the plasma;    -   comparing the color of the dye of the indicator with a first        reference color, characteristic of a reference temperature in        the chamber; and    -   comparing the color of the dye of the indicator with a second        reference color, characteristic of a reference quantity of atoms        of oxygen O and/or nitrogen N.

It is thus possible to compare the color of the dye at the beginning ofsterilization with a first reference color in order to ensure that thereference temperature in the post-discharge chamber has been reached.The color of the dye is also compared at the end of sterilization with asecond reference color, in order to ensure that the reference quantityof atoms of nitrogen N and/or oxygen O that have come into contact withthe indicator is reached during the sterilization process.

The invention also provides a sterilization indicator enablingverification of the temperature in the post-discharge chamber to besimplified, while also making it possible to verify the presence ofatoms of nitrogen N and/or oxygen O in the post-discharge chamber. Thisindicator comprises:

-   -   a first portion, comprising a compound of the heating type on        being put into contact with atoms of oxygen O and/or nitrogen N,        and a thermochromic dye in thermal contact with the compound;        and    -   a second portion, comprising a thermochromic dye, and not        including a compound of the heating type when put into contact        with atoms of oxygen O and/or nitrogen N.

The first portion is identical to the above-described indicator andserves to verify the content in atoms of nitrogen N and/or oxygen O inthe post-discharge plasma.

The second portion comprises a thermochromic dye, but does not include acompound of the heating type. The second portion of the sterilizationindicator serves to verify the temperature that exists inside thepost-discharge chamber (the sterilization chamber). The dye is not incontact with the heating compound, and it is subjected only to thetemperature of the post-discharge chamber, without any other heating.

The dye of the second portion of the indicator may be the same dye asthat present in the first portion.

The invention thus provides a method of indicating: a) the presence ofatoms of oxygen O and/or nitrogen N in a plasma present in apost-discharge chamber, in particular obtained from a gas containingoxygen O₂ and/or nitrogen N₂; and b) the temperature in thepost-discharge chamber. The method comprises in succession:

-   -   putting said sterilization indicator into contact with the        plasma; and    -   comparing the colors of the dyes of the first and second        portions of the indicator with two reference colors.

The color of the dye of the first portion of the indicator can becompared with the first reference color, characteristic of a referencequantity of atoms of oxygen O and/or nitrogen N, and then the color ofthe dye of the second portion of the indicator can be compared with thesecond reference color, characteristic of a reference temperature in thechamber. It is also possible to compare the colors of the two portionswith each other.

By using the two-portion indicator, there is no longer any need to trackchanges in the method in real time.

Other objects, characteristics, and advantages of the invention appearon reading the following description given purely by way of example, andmade with reference to the accompanying drawing, in which:

FIG. 1 is a diagram showing a sterilization device with an indicator ofthe invention;

FIG. 2 shows the indicator in a first embodiment of the invention; and

FIG. 3 is a plan view of an indicator in a second embodiment.

The sterilization device as shown in FIG. 1 comprises an inlet pipe 1for a stream of nitrogen N₂ that passes through an evacuated enclosure 2subjected to an electric field produced by a microwave generator 3operating at 2.45 gigahertz (GHz). The electric field serves inparticular to form atoms of N from the molecules of N₂. The plasma asproduced in this way is conveyed to a sterilization chamber 4 via a pipe5, with the help of a vacuum pump 6. The chamber 4 is fed with plasmavia a nozzle 15. The nozzle 15 may advantageously be terminated by oneor more injectors serving to make the plasma stream uniform. The vacuumpump 6 also serves to evacuate the plasma to the outside via a pipe 7provided with filters 8.

The sterilization chamber 4 is referred to as a “post-discharge” chambersince the plasma is not subjected to an electric field therein, beingsubjected to the field in the enclosure 2. The plasma present in thechamber 4, referred to as a “post-discharge” plasma, that is notsubjected to an electric field, thus no longer contains any ultraviolet(UV) radiation, any ions, or any electrons, thereby making it possibleto avoid excessively raising the temperature inside the chamber 4, whichwould damage the articles 10 that are to be sterilized.

The sterilization chamber 4 is in the form of a rectangularparallelepiped, and includes a metal or non-metal instrument carrier 9for receiving the articles 10 to be sterilized. The sterilizationchamber 4 is provided with heater means 11 delivering a temperature thatis controlled by a control device 12. These heater means can beconstituted, in particular by an electrical resistor or by inductanceheater means.

The sterilization chamber 4 is closed on one of its sides by a pivotaldoor 13.

The chamber 4 is also provided with a reflector 14 and a fan 16 thatcontributes to making the plasma uniform.

Articles 10 for sterilizing, together with one or more sterilizationindicators 17, are introduced into the chamber 4. The sterilizationindicators 17 may be placed throughout the chamber 4, and in particularin locations that are difficult to access, such as against its walls orunder the articles 10 to be sterilized. In this manner, it is possibleto ensure that the entire volume of the sterilization chamber is treatedby the nitrogen plasma.

It is also particularly useful to place sterilization indicators 17 onsterilization pockets containing the articles 10 to be sterilized, orinside the articles, in order to ensure that the articles 10 themselveshave been in contact with the atoms of nitrogen N.

The pressure that exists inside the sterilization chamber 4 ispreferably less than 10⁵ pascals (Pa), so as to facilitate putting atomsof nitrogen N into contact with the articles 10.

FIG. 2 shows a sterilization indicator 17 of the invention.

The sterilization indicator 17 comprises a medium 18 that is impregnatedwith a dye 19 that diffuses in the medium 18 to form a dye zone 20. Acompound 21, in the form of metal fibers, is placed in the medium and inthe dye zone 20. As shown in FIG. 2, the fibers are disposed within thedye zone 20, but it is also possible to envisage the fibers beingdispersed over the entire medium 18, thus extending beyond the dye zone20.

FIG. 3 shows a sterilization indicator 17 of the invention in twoportions. The indicator 17 comprises a first portion 22 comprising a dye19 that has diffused in the medium 18 to form a dye zone 20. A compound21 in the form of metal fibers is disposed in the medium in the dye zone20.

In the second portion 23 of the indicator 17, which does not include thecompound 21, the dye 19 forms a dye zone 24. The two dye zones 20 and 24are far enough apart for the metal fibers 21 not to be in thermalcontact with the dye zone 24 of the second portion 23.

In the two examples below, the parameters of the sterilization processare as follows: the temperature in the sterilization chamber 4 is 60°C., a nitrogen flow rate is delivered at 1 liter per minute (L/min),using nitrogen at a pressure of 6.66×10² Pa (5 Torr), and with anexposure time to the post-discharge plasma of 40 min.

Furthermore, the dye 19 used is the Kromagen magenta 120 screen ink dye.

The color of the dye varies as a function of temperature as follows:

20° C.: pale whitey pink

60° C.: pale pink

70° C.: pink

90° C.: magenta

120° C.: purple

With the help of various reference tests, it has been determined thatthe heating of the metal in the indicators of Examples 1 and 2 needs tobring the dye up to a temperature of 90° C. if the above-indicatedparameters are complied with.

EXAMPLE 1 A One-Portion Sterilization Indicator

The medium 18 was constituted by meta-aramid fibers.

The compound 21 was constituted by copper nickel alloy filaments havinga diameter of 20 micrometers (μm) as sold under the name Monel® by thesupplier Baltec Ltd. One way of fabricating the indicator consists inputting the filaments 21 into contact with the medium 18, and thenpouring the dye 19 onto the medium 18, and in allowing it to dry at 20°C. The indicator 17 as made in this way presents a pale whitey pinkcolor.

The indicator 17 was placed in the chamber 4 where it was subjected tothe sterilization process.

The dye 19 of the indicator 17 was observed after 1 minute ofsterilization. Its color was pale pink, showing that the temperature inthe chamber 4 was substantially 60° C.

At the end of the process, the indicator 17 was of a magenta color (darkpink), i.e. it had the desired color.

EXAMPLE 2 A Two-Portion Sterilization Indicator

The medium 18 was constituted by cotton fibers.

One way of fabricating the indicator consists in mixing copper filamentswith a diameter of 20 μm with cotton fibers in the first portion 22 ofthe indicator 17, and then in spreading the dye 19 on the medium 18,both in the first portion 22 and in the second portion 23 so as to formtwo dye zones 20 and 24, and leaving it to dry at 20° C. The indicator17 as fabricated in this way presents a pale whitey pink color.

At the end of the process, the dye zone 24 was pale pink, showing thatthe temperature in the chamber 4 was substantially 60° C. The dye zone20 was of magenta color (dark pink). The test was thus valid.

Thus, the color of the dye in contact with the metal is determined bytaking into account the various parameters that it is desired to useduring the sterilization process, such as the concentration of atoms ofnitrogen N in the plasma, the duration of the exposure of the articles10 to the plasma, the temperature that exists inside the sterilizationchamber 4, and the volume of the articles 10. The concentration in atomsof nitrogen N can be selected by adjusting the power of the microwavegenerator 3 and also the flow rate of nitrogen.

The determination of the color of the dye also takes account of thenature of the metal 21, since the heating of the surface of the metal isa function of the nature of the metal, as well as of the quantity ofatoms of nitrogen N in contact with the metal 21.

Thus, by means of the indicator 17 of the invention, it is possible tovalidate two parameters of the sterilization process, namely thequantity of atoms of nitrogen N and the sterilization temperature.

It can be decided to determine the sterility assurance level of theprocess as a function of the pressure and of the flow rate of nitrogen,of the content in terms of atoms of nitrogen N, of the temperature inthe sterilization chamber 4, and of the duration of sterilization. Sincethe density of atoms of nitrogen N is proportional to the power of themicrowave generator 3, at given nitrogen pressure and flow rate, theparameters for verification are temperature, content of atoms ofnitrogen N, and duration of the treatment.

The sterilization indicator 17 of the invention is thus particularlysuitable for ensuring that the atoms of nitrogen N present in thepost-discharge plasma have indeed come into contact with all of thesterilization chamber, as well as with the articles 10 to be sterilized.The indicator 17 also functions with post-discharge plasmas obtainedfrom gas comprising a mixture of N₂ with other species, such as theplasmas obtained from N₂/H₂ or Ar/N₂.

The indicator 17 also functions with atoms of oxygen O present in apost-discharge plasma obtained from a gas including oxygen O₂, such asplasmas obtained from N₂/O₂ or Ar/O₂.

1-16. (canceled)
 17. A sterilization indicator comprising: a compoundthat is of the heating type when put into contact with atoms of oxygen Oand/or nitrogen N; and a thermochromic dye in thermal contact with thecompound.
 18. The indicator according to claim 17, wherein the compoundcomprises at least one metal.
 19. The indicator according to claim 18,wherein the metal is selected from the group consisting of copper,titanium, steels, aluminum, and alloys thereof.
 20. The indicatoraccording to claim 19, wherein the metal is selected from the groupconsisting of copper and alloys thereof.
 21. The indicator according toclaim 17, wherein the compound is in the form of a powder, particles,filaments, or fibers.
 22. The indicator according to claim 17, whereinthe compound and the dye are in contact with a medium.
 23. The indicatoraccording to claim 22, wherein the medium is in the form of a plate orin the form of fibers.
 24. The indicator according to claim 23, whereinthe medium is selected from the group consisting of cellulose, fabrics,cotton, paper, and blotting paper.
 25. The indicator according to claim17, wherein the dye is an irreversible thermochromic dye.
 26. Theindicator according to claim 17, comprising: a first portion, comprisingsaid compound of the heating type on being put into contact with atomsof oxygen O and/or nitrogen N, and said thermochromic dye in thermalcontact with the compound; and a second portion, comprising athermochromic dye, and not including a compound of the heating type whenput into contact with atoms of oxygen O and/or nitrogen N.
 27. The useof an indicator according to claim 17 in a sterilization device.
 28. Theuse according to claim 27, for indicating the presence of atoms ofoxygen O and/or nitrogen N in a post-discharge plasma.
 29. The use of anindicator according to claim 26 for indicating in addition thetemperature in a post-discharge chamber.
 30. A method of indicating thepresence of atoms of oxygen O and/or nitrogen N in a plasma present in apost-discharge chamber, the method comprising: putting a sterilizationindicator according to claim 17 into contact with the plasma; and thencomparing the color of the dye of the indicator with a reference color.31. A method of indicating: a) the presence of atoms of oxygen O and/ornitrogen N in a plasma present in a post-discharge chamber; and b) thetemperature in the post-discharge chamber, comprising the steps: puttinga sterilization indicator according to claim 17 into contact with theplasma; and then comparing the color of the dye of the indicator with afirst reference color characteristic of a reference temperature in thechamber; and then comparing the color of the dye of the indicator with asecond referenced color characteristic of a reference quantity of atomsof oxygen O and/or nitrogen N.
 32. A method of indicating: a) thepresence of atoms of oxygen O and/or nitrogen N in a plasma present in apost-discharge chamber (4); and b) the temperature in the post-dischargechamber, comprising the steps: putting a sterilization indicatoraccording to claim 26 into contact with the plasma; and then comparingthe colors of the dyes of the first and second portions of the indicatorwith two reference colors.